Boat sterns, especially on more sporty boats, have always shown great diversity, with configurations such as vertical, forward- or aft-raked, rounded, gently curved, pointed and the like. It has therefore been customary up to now when fitting self-steering systems, bathing ladders or other accessories to the sterns of such boats to insert a section of hardwood or the like, specially shaped to match the configuration of the stern, between the retaining component of the accessory and the stern and to mount and fix the accessory with screw bolts that pass through the accessory retaining component, the wooden spacer and appropriate holes in the transom to washers and nuts inside the boat. Preparing wooden spacers of this type is a time-consuming exercise and even if the spacer is exactly matched to the contour of the transom, the accessory retaining component will only be able to establish a tight, stress-free fit against the stern of the boat if the accessory is ultimately mounted in precisely that position on the stern for which the wooden spacer was prepared. The root of this problem, then, is that not only are most boat sterns shaped in three-dimensions, but also the progression of the shaping changes in the horizontal and vertical directions.
Considerable stress problems normally occur in any case even where the wooden spacer does allow the accessory retaining component to fit tightly against the stern of the boat. This is because the screw bolts for the accessory mounting device are all oriented approximately parallel with one another and sit with their heads resting against flat sections of the surface of the retaining element. Wooden spacers are prepared primarily to level out the horizontal and vertical curvature of the stern and provide a flat surface for the accessory retaining element. In consequence, most retaining bolts pass through the transom of the boat at an oblique rather than a right angle, and there must therefore be another levelling spacer arranged inside the boat between the locknuts and the transom to ensure that the whole pressure surface of each locknut is able to rest evenly against the inside surface of the stern of the boat.